Manufacture of brazed finned tubing and the like



Sept. 24, 1957 R. w. scHRoEDER MANUFACTURE oF BRAZED FINNED TUBING ANDTHE LIKE BY I ATTORNEYS United States Patent O 'MANUFACTURE or BRAZEDFINNED TUBING AND THE Luni Robert W. Schroeder, Massillon, Ohio,assignor to The Griscom-Russell Company, Massillon, Ohio, a corporationof Delaware Application September 27, 1955, Serial No. 536,831

7 claims. (Cl. zes-157.3)

The invention relates to the manufacture of finned tubing suitable foruse in heat exchangers and other purposes, and more particularly to amethod for expeditiously and economically manufacturing brazed nnedtubingwhereby the fins are bonded vto the, tubing along the entireinterface.

In use, such finned tubing is subjected to extremely high temperaturesand high pressures Vand is frequently exposed to highly corrosiveiiuids. The tube must therefore necessarily be formed'of a hightemperature resistant, high strength and corrosion-resisting material.such as stainless steel. For the purposes of obtaining the maximum heatexchange, the fins should be formed of a high heat conducting materialsuch as copper.

It is important that the exterior surfaces of such a copper fin beprotected against the corrosive action of iiuids to which they areexposed. For this purpose stainless steel may be clad'pupon the copperfin. The outer edge of the lin may be protected by using a copper finclad on only one side with stainless steel and folding' the n materialupon itself and attaching the edges of the folded fin to the tube.

However, in many instances the finned tube may comprise an unfolded finof single thickness, in which the copper fin is clad on both sides withstainless steel. Thus, the outer edge of the copper is exposed. Theinvention contemplates the protection of this outer edge of the fin bybuilding up a deposit of brazing alloy thereon to entirely seal andprotect the copper.

Two types of fins are commonly used in the construction of such finnedtubing. One type of fin comprises straight, flat ribbons extendinglongitudinally of the tube and attached thereto in radially disposedposition. The other type of fin comprises a ribbon spirally wound aroundthe tube, with the inner edge thereof seated in a spiral groove in thetube or contacting the periphery thereof. The spirally wound n may be.Very thin, say .008" to .010" thick, and there may be as many as twentyto thirty winds per inch of tube length.

While the present invention is applicable to either of these types offinned tubes, it is illustrated and described herein as applied to thespiral fin type to which it is especially adapted. These spiral ns arestressed and stretched as they are wound around the tube, which tends tohold the n upon the tube.

If the fin stress is released, the bond between the fin and tube becomesless secure. A differential in expansion of the two materials mayaggravate these conditions,

causing the bond between the fin and tube to be destroyed.

In the manufacture of such finned tubing for use in heat exchangers itis desirable that this bond be obtained and maintained during use athigh temperature, as other- Wise the conductivity is reduced,proportionately decreas-l ,of brazing alloy upon the tube at and aroundthe base of the iin so as, after subsequent brazing, to obtain and "icemaintain a tight secure bond between the tube and the lin under allconditions to which the nned tube may be subjected in normal use. y

Likewise, where the fin `is formed of a copper ribbon clad on both sideswith stainless steel, with the inner edge thereof bonded to the tube,the outer edge of the copper fin may be sealed and protected againstcorrosive fluids by building up a deposit of brazing alloy thereon. Inorder to facilitate adherence of the brazing alloy, the fin tip may beroughened by shot or grit blasting or by means of a knurled roll. Theexterior surface of the tube may, in some instances, require similarroughening in'order to facilitate adherence of the brazing alloy to thetube and the base of the fin tip. Y

It is therefore a primary object of the invention to provide a method`of expeditiously and economically manufacturing brazed finned tubingwhereby a tight andrsecure bond is obtained'and maintained between thefin and tubing along the entire interface. Y f

Another object is to provide such a method by which the brazing alloy iscaused `to flow into all small crevices between the tube and the fin.

A further object is to provide` a method of manufacturing brazedfinnedtubing which includes heating of the tube and then, sprayingpowdered brazing alloy onto desired portions of the heated parts'so thatit will adhere thereto as a sintered deposit, and then furnace brazingthe finned tube to cause the brazing alloy to ow over desired areas andalso into all fine crevices between the tube and fin.

A still further object is to provide for the manufacture lof brazedfinned tubing by placing a finned tube in a lathe and locating anoxy-acetylenetorch in the tool post of the lathe so that the flamethereof will impinge upon the tube and the fin, rotating the finned tubeand simultaneously moving the torch from one end of the n to a pointspaced therefrom to preheat the adjacent end portions of the tube andfin, then backing up the torch to the starting point, and then sprayingpowdered brazing alloy from the torch'upon the tube and fin as the torchis continuously moved toward'the other end of the n.

Another important object `of the invention is to provide a method ofprotecting the outer edge of the fin by depositing brazing alloy thereonat the same time that the brazing alloy is deposited on the tube at andaround the base of the iin. l

A further object is to provide for protecting the outer edge of the finby first rougheningsaid outer edge, then depositing brazing alloy uponthe roughened outer edge of the fin, and then melting and flowing thedeposited alloy.

A still further object is to provide such a method of manufacturingbrazed vfinned tubing, which consists in applying a sintered deposit ofpowdered brazing alloyto a nned tube in the manner referred to, and thenfurnace brazing the finned tube to cause the brazing alloy to fiowoutward on the iin and into fine crevices between the tube and fin. v v

These and other objects which will be apparentrto those skilled in theart, or which lmay be later pointed out, may be yaccomplished by themethods, constructions, arrangements, partsgcombinations `andsub-combinations comprising the present invention, the nature of whichis set forth in the following general statement, a preferred embodimentof whichillustrative of the best mode in which applicant hascontemplated applying the principles -is set forth in the followingdescription. and illustrated in the accompanying drawing, and which isparticularly and distinctly pointed out and set forth in the appendeclaims forming part hereof. Y

The method and apparatus comprising the invention provided either withfins wound spirally around the tubes or fins extending longitudinally ofthe tube. For the purpose of illustration, the invention is shown anddescribed as applied to finned tubing of the spiral fin type, and thefollowing general statement brieliy describes the manner in which theinvention may be carried out.

In general terms, the invention comprises a method of and apparatus forapplying brazing alloy to a tube having a fin spirally wound around theouter surface thereof, so as to securely bond the base of the fin to thetube and also to seal and protect the outer edges of the fin. Theinvention is carried out by rotating the finned tube and heating one endportion thereof, and then spraying powdered brazing alloy upon therotating tube to apply a sintered deposit of brazing alloy thereto atthe base of the fin and upon the outer edge of the fin, by a torch movedfrom the preheated end of the tube to the other end thereof, the tubeand fin being heated by the torch ahead of the application of thebrazing alloy.

This may be accomplished upon a lathe, the finned tube being mounted forrotation between the head stock and tail stock of the lathe, and thetorch being mounted upon theV tool post. A torch is used from which adesired powdered metallic brazing alloy is blown by compressed air ontothe work, with and through an oxyacetylene iiame which heats themetallic powder and deposits the same on the work on which it adheres insintered condition.

The method is performed by first using the torch without the metallicbrazing powder, to preheat one end portion of the rotating finned tubeto the desired temperature. The tool post, upon which the torch ismounted, is then backed up to the starting point and powdered brazingalloy is fed to the torch, which is then advanced along the length ofthe tube by the usual lead screw which operates the tool post of thelathe. A deposit of sintered brazing alloy is thus sprayed upon andadheres to the rotating tube at and around the base of the lin, and uponthe outer edges of the fin so that a sintered deposit of brazing alloyis applied to these portions throughout the entire finned length of thetube.

This manner of application of the brazing powder avoids -any plugging ofthe brazing alloy between fins which might occur by dipping or painting.In cases where the fin is copper clad material, it is especiallyimportant that any free or exposed copper edge be sealed. For thispurpose the metallic brazing alloy powder is deposited by the torch onthe outer edge of the clad fin, and, if necessary to promote adherence,the outer edge of the clad fin may be ronghened, as by shot or gritblasting, or by a knurled roller or the like, so that the sintereddeposit of brazing alloy will adhere thereto.

The finned tube, with brazing alloy deposited thereon in this manner, isthen heated to brazing temperature in a special atmosphere brazingfurnace to cause the brazing alloy to flow into all fine crevicesbetween the tube and fin, and to flow upward around-the base portions ofthe fin, and over the outer edge of the fin, forming a tight', unbrokenbond between the tube and the fin, and also a protective coating sealingand protecting the `outer edge of the iin with a securely bonded depositof brazing alloy.

Having thus described the invention in general terms, by way of examplean embodiment of `the invention is illustrated in the accompanyingdrawing forming a part hereof, wherein like numerals indicate similarparts throughout the several Views, and in which;

Fig. 1 is adiagrammatic front elevation of a llathe upon which theinvention may be performed;

Fig. 2 is a top plan view of the lathe shown in Fig. 1;

Fig. 3 is an enlarged, fragmentary, sectional view of a portion of afinned tube showing the sintered deposit of brazing alloy applied to thetube at the base portions of the fin and also upon the outer edges ofthe fin;

Fig. 4 is a fragmentary, sectional view of a further en- Cit 4., largedportion of the finned tube shown in Fig. 3, showing-the brazing alloyflowed outward upon the fin and into crevices between the fin and tube,after the furnace brazing operation;

Fig. 5 is a view similar to Fig. 4, of a slightly modified form of thefinned tube after the furnace brazing; and,

Fig. 6 is a greatly enlarged fragmentary, sectional view through the tipor outer edge portion of a clad fin showing the manner in which the sameis sealed and protected by la coating of brazing alloy.

Referring first more particularly to Figs. l and 2, the lathe upon whichthe invention may be carried out comprises generally the bed 10 havinghead stock 11 at one end provided with a rotatable chuck 12 ofconventional design, adapted to be rotated in a usual and well knownmanner by conventional gearing and the like (not shown) driven by anysuitable power means.

The tail stock 13 is mounted at the other end of the lathe, and a toolpost 14 is mounted for longitudinal movement upon the bed of the latheby the usual lead screw (not shown) coordinated with the mechanism whichrotates the chuck in the head stock, as in usual practice.

The lathe differs from conventional lathe construction only in that atorch indicated at 15 is substituted upon the tool post 14 instead ofthe usual tool. The torch is of the type into which powdered metal maybe blown by compressed air through an oxyacetylene flame causing themetal to be heated and deposited on the work in a sintered state.

The finned tube to be brazed comprises the tube, indicated generally at16, which may be formed of stainless steel, ferrous metal, nickel,Inconel or the like and upon which is spirally wound a fin, indicated at17, which may be formed of copper clad with stainless steel, or the finmay be formed of ferrous metal, nickel or Inconel.

As shown in Figs. 3V and 4, the base portions of the fin may be locatedin a spiral groove 18 formed in the peripheral surface of the tube 16,into which the fin is wound under stress, or, as shown in Fig. 5, thefin 17 may be spirally wound, under tension, around the peripheralsurface of the tube.

These spiral fins are stressed and stretched as they are spirally woundyunder tension around the tube, the stress holding the lin upon thetube. It is evident that if the fin stress is released, the blondbetween the fin and tube will be at least partially, Vif not entirelydestroyed. These conditions may be aggravated in use by a differentialin expansion of the two materials.

In either case, there will be fine crevices between the base portions ofthe iin and the tube. It is highly desirable that the brazing alloy whenheated sho-uld flow into these fine crevices in order to insure a securetight bond at these points so as to pro-vide proper heat conductivitybetween the fin and tube when used in a heat exchanger, as well as toresist the corrosive action of uids to which the finned tube may besubjected in use.

Heretofore, it has not been possible tosatisfactorily Viiow brazingalloy into such lfine erevices, and as a result a tight bond could notbe obtained and maintained between the tube and the base portions of thefin.

I have discovered that by heating the tube and fin and immediatelyapplying a sintered deposit .of brazing alloy thereto, that the powderedmetallic brazing alloy would adhere to these heated surfaces, and whenthe finned tube was subsequently heated in a brazing furnace, thedeposited alloy would melt and flow into fine crevices, producing'atight bond between the tube and the base portions of the fin.

After the fin has 'been attached to the tube, the finned tube is mounted`in the lathe, between the head stock and tail stock as shown 'in Figs.l and 2, with the tool post located in the starting position, adjacentto the left hand end of the finned portion of the tube, as viewed inFigs. l and 2.

The torch 15 -is operated, without the application of powdered metal, soas to direct the ame upon the finned tube, and the lathe mechanism isstarted to rotate the finned tube and simultaneously move the tool postcarrying the torch toward the right as viewed in Figs. 1 and 2, heatingthe tube and iin to desired temperature.

When the torch reaches a position spaced from the starting end,approximately such as shown in full lines in Figs. 1 and 2, the toolpost is backed up to the starting point and powdered brazing alloy isfed to the torch.

The lathe is then operated to rotate the finned tube and continuouslymove the tool post, carrying the torch, toward the iight, depositingsintered brazing alloy Aupon the tube 16 adjacent to the base of the finas indicated at 19 in Fig. 3 and also upon the outer edgeof the iin asindicated at 20, the sintered alloy adhering to the heated surfaces ofthe tu-be and fin.

The sintered brazing alloy is thus deposited upon the hot surfaces ofthe tube and iin, and, .as ,the torch continues to move toward theright,each successive portion of the tube and fin will be heated by the torchflame ahead of the brazing alloy applied thereto, so that the sinteredalloy will adhere to the tube at and around the base of the fin, and tothe outer edge of the n, throughout the length thereof.

Thus, all portions of the tube and lin, from one end thereof to theother, will be heated to the necessarytemperature before the brazingalloy is deposited thereon, thus causing the sintered alloy to adhere tothe tube and fin material, even though the iinmaterial is very thin, say.008 in thickness and spaced say 20 to 30 convolutions per inch of tubelength. Moreover, the deposit f the brazing powder in this manner avoidsany plugging of the extremely narrow spaces between iin convolutions.

In Fig. 6 is shown a greatly enlarged section of the outer edge or tipportion of a stainless steel clad copper fin, showing the manner inwhich the brazing alloy deposited upon the outer edge of the iin sealsand protects the copper.

The copper iin is indicated at 21 and the stainless steel clad Vuponeach side thereof is indicated at 22. As shown in Fig. 3, the brazingalloy 20 deposited upon the outer edge of the fin entirely covers andseals the edge of the copper fin 20 and extends over the edges of thestainless steel clad layers 22.

The outer edges of these stainless steel layers may be slightly rounded,as shown at 23, causing the brazing alloy deposit to flow downwardthereover at eac'h side of the iin, during the furnace brazingoperation, thus comipletely sealing and protecting the copper.

ln order to assure adherence of the deposit of brazing alloy to theouter edge of the iin, the outer edge may be roughened by shot or gritblasting, knurling or the like. If desired, the tube surfaces may besimilarly roughened so as to assure adherence of the brazing alloythereto adjacent the base of the iin.

The brazing alloy may be a commercial alloy of suitable compositionincluding elements such as nickel, lchromium, iron, silicon and boron.After the brazing, alloy has been deposited upon the tube at the base ofthe fin, and upon the outer edge of the iin, as shown in Fig. 3, in themanner above described, the brazed iinned tube is then furnace brazed,with brazing temperatures and atmospheres appropriate to the type ofalloy being used.

This furnace brazing is carried out at a temperature above the flowpoint of the brazing alloy but below the melting point of the copper inthe iin so as to cause the brazing alloy to ow upward or outward uponthe base portion of the fin, as indicated at 24 in Figs. 4 and 5, andinto all iin crevices between the iin and the tube such as the grooves18 in Fig. 4 and the crevices 25 between the base of the fin and thetube in Fig. 5. The iin will thus be tightly bonded to the tube at allpoints throughout its length, Y

As above poi-nted out, in nned tubes of this character the iin is heldonto the tube becausethe iin mateesoneri.

rial is stressed as it is wound onto the tube. If the tin stress isreleased the bond between the iin and the tube becomes less sure.Differential in expansion of the materials may aggravate this condition.By applying the brazing alloy in the manner above described, a sure`bond between the fin and the tube is obtained and maintained underworking conditions.

Although the invention has been illustrated and described in detail asapplied to the manufacture of finned tubes with fins spirally woundthereon, it should be understood that tubes having fins extendinglongitudinally thereof may be brazed in the same manner.

In the foregoing description, certain terms have been used for brevity,clearness and understanding, but no unnecessary limitations are to beimplied therefrom beyond the requirements of the prior art, because suchwonds are used for descriptive purposes herein and are intended to bebroadly construed. n

Moreover, the embodiments of the improved construction illustrated anddescribed herein are by way of example, and the scope of the presentinvention is not limited to the exact details of construction.

Having now described the invention or discovery, the construction, theoperation, and use of preferred embodiments thereof, and theadvantageous new and useful results obtained thereby; the new and usefulconstruction, and reasonable mechanical equivalents thereof obvious tothose skilled in the art, are set forth in the appended claims.

I claim:

v1.' The'methodof making finned tubes for heat exchangers of a typeincluding a corrosion-resistant tube having high-temperature strengthand iin ribbon means bonded thereto formed of high-heat-conductivitymaterial having corrosion-resistant surfaces, which includes the stepsof providing a stainless steel tube; `Securing copper ribbon iin meanshaving stainless steel clad to both ribbon surfaces of the iin, to theexterior of the tube with the base edge of the iin ribbon in contactwith the tube and with copper exposed' at the outer edge of then'ribbon; rotating the tube with the lin means secured thereon on itsaxis; initially preheating one end of the rotating tube and n means;directing a flame and a spray of heated powdered metallic brazing alloysimultaneously against the heated end of the rotating tube and iinmeans; moving the Haine and spray axially of the rotating tube and linmeans from said heated end to the other end of the tube; such directingand` moving of the flame and spray with respect to the rotating tube andiin means preheating successive unheated tube and fin means portionsbeyond the initially preheated rotating one end of the tube and finmeans and ahead of the spray; progressively applying, by said directedand moving spray, an adherent deposit of heated powdered alloy metal onthe heated tube at and around the base edge of the iin ribbon and on thecopper exposed outer edge of the iin'ribbon throughout the length of therotating tube and fin means; furnace heating the tube and 1in means withthe adherent deposit thereon to brazing temperature; melting theadherent deposited alloy, by said furnace heating, and flowing themelted alloy outward on the iin surfaces from the base edge of the iinand also along the contacting interface surfaces of the base edge of thefin and tube and also over the outer edge of the tin; and the melted andflowed alloy metal forming a tight unbroken bond between the tube andfin means and protecting and sealing the outer exposed copper edge ofthe fin means.

2. The lmethod of making finned tubes for heat exchangers of a typeincluding a corrosion-resistant tube having high-temperature strengthand iin ribbon means bonded thereto formed of high-heat-conductivitymaterial having corrosion-resistant surfaces, which includes the stepsrof providing a stainless steel tube; spiralling winding a `copper ribbonhaving stainless steel clad on both ribbon surfaces around the tubeunder ltension to hold the tin upon the tube with the base edge of thetin ribbon in contact with the tube and with copper exposed at the outeredge of the tin ribbon; rotating the tube with thespiral iin heldthereon on its axis; initially preheating one end of the rotating tubeand fin; directing a flame and a spray of heated powdered metallicbrazing alloy simultaneously against the heated end of the rotating tubeand fin; moving the llame and spray axially of ythe rotating tube andiin from said heated end to the other end of the tube; such directingand moving of the flame and spray with respect to the rotating tube andn preheating successive unheated tube and fin portions beyond theinitially preheated rotating one end of the tube and n and ahead of thespray; progressively applying, by said directed and moving spray, anadherent deposit of heated powdered alloy metal on the heated tube atand around the base edge of the iin ribbon and on the copper exposedouter edge of the fin ribbon throughout the length of the rotating tubeand iin; furnace heating the tube and iin with the adherent depositthereon to brazing temperature; melting the adherent deposited alloy, bysaid furnace heating, and flowing the melted alloy outward on the iinsurfaces from the base edge of the fin and also along the contactinginterface `surfaces of the base edge of the tin and tube and also overthe outer edge of the fin; and the melted and flowed alloy metal forminga tight unbroken bond between the tube and n and protecting and sealingthe outer exposed copper edge of the n.

l3. The method of making finned tubes for heat exchangers of a typeincluding a corrosion-resistant tube having high-temperature strengthand n ribbon means bonded thereto formed of highheatconductivitymaterial having corrosion-resistant surfaces, which includes the stepsof providing a corrosion-resistant tube; securing fin ribbon meansformed of high-heat-conductivity material having,y corrosion-resistant`surfaces to the exterior of the tube with the base edge of the iinribbon in contact with the tube and with the high-heat-conductivity finmaterial exposed at the outer edge of the n ribbon; rotating the tubewith the tin means secured thereon on its axis; initially preheating oneend of the Arotating tube and iin means; directing a flame and a sprayof heated powdered metallic brazing alloy simultaneously against theheated end of the rotating tube and n means; movingthe flame and sprayaxially of the rotating tube and n means from said heated end to theother end of the tube; such directing and moving or" the flame and spraywith respect to the rotating tube and n means preheating successiveunheated tube and tin means portions beyond the initially preheatedrotating one end of the tube and lin means and ahead of the spray;progressively applying, by said directed and moving spray, anradherentdeposit of heated powdered alloy metal on the heated tube at and aroundthe base edge of the tin ribbon and on the exposed outer edge of the tinribbon throughout the length of thc rotating tube and iin means; furnaceheatingthe tube and tin means with the adherent deposit thereon tobrazing temperature; melting the adherent deposited alloy, by saidfurnace heating, and owing the melted alloy outward on the tin surfacesfrom the base edge of the fin and along the contacting interfacesurfaces of the base edge ofthe lin and tube and also over the outeredge of the tin; and the melted and owed alloy metal forming a tightunbroken bond between the tube and iin means and protecting and sealingthe outer exposed edge of the fin means.

4. The method of making nned tubes for heat exchangers of a typeincluding a corrosion-resistant tube having high-temperature strengthand fin ribbon means bonded thereto formed of high-heat-conductivitymaterial having corrosion-resistant surfaces, which includes the stepsof providing a heat-resistant metal tube, securing iin ribbon meansformed of highheat=conductivity material to the exterior of the tubewith the base edge of the tin ribbon in contact with the tube; rotatingthe tube with the n means secured thereon on its axis; initiallypreheating one end of the rotating tube and tin means; directing aHarrie and a spray of heatedpowdered metallic bra'zing alloysimultaneously against the heated end of the rotating tube and n means;'moving the flame and spray axially of the rotating tube and n meansfrom said heated end to the other end of the tube; such directing andmoving of the flame and spray with respect to the rotating tube and tinmeans preheating successive uriheated tube and fin means portions beyondthe initially preheated rotating one end of the tube and tin means andahead of the spray; progressively applying, by said directed and movingspray, an adherent deposit of heated powdered alloy metal on the heatedtube at and around the base edge of the fin ribbon throughout the lengthof *the rotating tube and lin means; furnace heating the tube and finmeans with the adherent deposit thereon to brazing temperature; meltingthe adherent deposited alloy, by said furnace heating, and owing themelted alloy outward on the fin surfaces from the base edge of the tinand also along the contacting interface surfaces of the base edge of theiin and tube; and the melted and owed alloy metal forming a tightunbroken bond between the tube and fin means.

5. The method of making finned tubes for heat exchangers of a typeincluding a corrosion-resistant tube having high-temperature strengthand n ribbon means bonded thereto formed of l'iigh-heat-conductivitymaterial having corrosion-resistant surfaces, which includes the stepsof providing a stainless steel tube; securing copper ribbon n meanshaving stainless steel clad to both ribbon surfaces of the fin, to theexterior of the tube with the base edge of the iin ribbon in contactwith the tube and with copper exposed at the outer edge of the finribbon; rotating the tube with the iin means secured thereon on itsaxis; directing a ame and a spray of heated powdered metallic brazingalloy simultaneously against one end of the rotating tube and n means;moving the llame and spray axially of the rotating tube and iin meansfrom said one end to the other end of the tube; such directing andmoving of the flame and spray with respect to the rotating tube and finmeans preheating successive unheated tube and tin means portions aheadof the spray; progressively applying, by said directed and moving spray,an adherent deposit of heated powdered alloy metal on the tube heated bysaid flame at and around the base edge of the tin ribbon and on thecopper exposed outer edge of the fin ribbon throughout the length of therotating tube and tin means; furnace heating the tube and tin means withthe adherent deposit thereon to brazing temperature; melting theadherent deposited alloy, by said furnace heating, and flowing themelted alloy outward on the n surfaces from the base edge of the fin andalso along the contacting interface surfaces of the base edge of the nand tube and also over the outer edge of the tin; and the melted andflowed allo-y metal forming a tight unbroken bond between vthe tube andfin means and protecting and sealing the outer exposed copperedge of thetin means.

6,v The method of brazing a finned tube with a brazing alloy whichconsists in rotating the finned tube on its axis, directing a flame uponone end of the rotating finned tube, moving the ame axiallyof therotating tube to a point spaced from said one end thereby preheating thetube from said one end to said point, returning the fiamc to saidy oneend, directing said ame and a spray of heated metallic brazing alloypowder simultaneously against the heated tube at said one end, movingthe flame and heated powder spray axially of the rotating tube to theother end of the tube thereby preheating successive portions of thefinned tube ahead of the spray and depositing heated powder on theheated tube as an adherent metal coating adjacent the base of the finprogressively throughout the length of the heated finned tube, and thenfurnace heating the finned tube to brazing temperature and therebymelting and owing the adherent deposited metal along the iin 9 surfacesat the base of the iin and into interface crevices between the tube andn.

7. The method of brazing a finned tube with a brazing alloy whichconsists in rotating the finned tube on its axis, directing a ame uponone end of the rotating nned tube, moving the flame axially of therotating tube to a point spaced from said one end thereby preheating thetube from said one end to said point, returning the ame to said one end,directing said ame and a spray of heated metallic brazing alloy powdersimultaneously against the heated tube at said one end, moving the ameand heated powder spray axially of the rotating tube to the other end ofthe tube thereby preheating successive portions of the finned tube aheadof the spray and depositing heated powder on the heated tube as anadherent metal coating adjacent the base of the iin and upon the outeredge of the iin progressively throughout the length 10 of the heatedvfinned tube, and then furnace heating the nned tube to brazingtemperature and thereby melting and flowing the adherent coating metalalong the n surfaces at the base of the n and into interface crevicesbetween the tube and iin and over and around the outer edge of the fin.

References Cited in the tile of this patent UNITED STATES PATENTS1,256,599 Schoop Feb. 19, 1918 1,412,656 Jenkins Apr. 11, 1922 2,092,018Quarnstrom Sept. 7, 1937 2,164,737 Ford July 4, 1939 2,314,902 ShepardMar. 30, 1943 2,440,698 Patterson May 4, 1948

